Method for reducing lossiness of sheet metal

ABSTRACT

LOSSINESS OF GRAIN-ORIENTED IRON ALLOY SHEETS IS REDUCED BY PARTIAL PLASTIC DEFORMATION OF THE SURFACE ON THE SHEET, E.G. BY PROVIDING NARROWLY SPACED GROOVE, PREFERABLY AT DIFFERENT ORIENTATION IN OPPOSITE SIDES OF THE SHEET.

United States Patent 3,647,575 METHOD FOR REDUCING LOSSINESS 0F SHEET METAL Alfred Fiedler, Duisburg-Huclringen, and Werner Pepperholf, Duisburg, Germany, assignors to Mannesmann Aktiengesellschaft, Dusseldorf, Germany No Drawing. Filed Oct. 17, 1969, Ser. No. 867,406 Claims priority, application Germany, Oct. 17, 1968, P 18 04 208.5 Int. Cl. H01f 1/16 U.S. Cl. 148-111 9 Claims ABSTRACT OF THE DISCLOSURE Lossiness of grain-oriented iron alloy sheets is reduced by partial plastic deformation of the surface of the sheet, e.g., by providing narrowly spaced, shallow grooves, preferably at dilferent orientations in opposite sides of the sheet.

The present invention relates to a method for reducing the ohmic losses in sheets as they are used in electrical equipment. In particular, the invention relates to improvements in the making of sheets of silicon iron alloys or iron molybdenum or the like, whereby the improvement concerns particularly the electric power loss in equipment using such sheets, e.g. transformers.

Sheets with secondary recrystallization are known for use in electrical equipment in which the grains are singly oriented and have (110) [001] position, which is also called Goss-position. These types of sheets have been used for about 30 years. In the past ten years interest in doubly oriented iron alloy sheets has developed, whereby the metal secondarily recrystallizes in the (100) [001] position, also called cube position. Sheets having Goss-texture are used primarily in large transformers. These sheets have a direction of easy magnetization (and thus lowest losses) in the direction of rolling. However, sheets with recrystallization in cubic position have two directions of easy magnetization, the direction of rolling and the direction transverse thereto.

The ohmic losses of a sheet with cubed texture are generally somewhat higher for magnetization in direction of rolling, than losses in a sheet of singly oriented grains for magnetization in the same direction. Assuming the sheets have similar composition, there are two reasons for this difference in losses.

First, cube texture sheets recrystallize in grains or crystals which are considerably larger than in a singly oriented sheet. Thus, the cube texture sheets have large ferromagnetic domains so that in case of magnetization in an alternating field the Block walls obtain large speeds, which in turn results in larger eddy current losses. Consequently the overall ohmic losses are relatively high.

Second, the orientation of the individual crystallites in the sheet is not ideal, which is true for singly as well as in doubly oriented sheets. in particular, the cube edge or [001] direction of the crystals or grains along which magnetization is the easiest, deviates from the ideal position. One component of the deviation occurs in the plane of the sheet, and there is in particular some deviation of the [001] directions from the direction of rolling. This deviation is to both sides from the direction of rolling and can be considerable. Furthermore, the (100) surface of most crystals is rotated out of the surface of the sheet by a few degrees or more.

In case of singly oriented sheets the deviation of the (100) surface from ideal orientation is within five degrees or less of about 80 to 90% of the grains which deviation does not materially increase the ohmic losses of such a 3,647,575 Patented Mar. 7, 1972 Ice sheet. However, disorientation of the (100) surface relative to the surface of a doubly oriented sheet and amounting to one or two degrees only already causes considerable increase in ohmic losses. The surface of such a sheet has compensating areas or regions in a thin surface layer, having the form of the well-known pine tree or dendrite pattern and having walls reducing stray fields caused by the grain surface disorientation. Consequently there are additional rotational processes during remagnetization and particularly in a thin surface layer, which rotational processes impede remagnetization in addition to Bloch wall shifting, and this in turn increases the ohmic losses. Ohmic losses in cube texture sheets differ after production thereof and a considerable portion of the sheets had to be sorted out or even eliminated from further use. A small deviation of the [001] direction, or of its projection into the plane of the sheet, from the rolling direction, up to about five degrees, does not significantly increase the ohmic losses, even in cube texture sheets.

It is an object of the present invention to reduce lossiness of such sheets, particularly of those having cube texture, lossiness to refer to ohmic power losses when the sheets are used in electrical equipment. In accordance with the present invention it is suggested to provide partial plastic deformation of the already completely recrystallized sheet having coarse grain structure, for changing the domain structure of the individual crystallites. It is another feature of the invention to provide grooves, serrations or the like, by partial plastic deformation wherein the grooves have a distance from each other which is considerably smaller than the mean size of the oriented grains of the sheet. The groove distance should preferably be between 0.1 and 1.0 mm. It is not required to obtain deeper grooves, serrations, etc. than 40.10 depth.

It was found to be of particular advantage to orient the grooves to run parallel to each other and to the direction of rolling on one side of the sheet and to run parallel to each other but transverse to the direction of rolling on the other side. It is furthermore advisable to stress anneal the sheets during or after grooving to eliminate stress and tension.

It has become known that plastic deformation of magnetically soft material widens the hysteresis loop, i.e., the coercivity (and thus the ohmic losses in case of AC magnetization) is increased. For precisely that reason it is the general rule that even smallest scratches of the surface should be avoided during production of sheets having cube texture. It was thus surprising that such plastic deformation which is a considerable surface disturbance, does not only fail to increase losses but actually reduces them. Depending upon the crystal surface disorientation and, therefore, depending upon the losses to be expected, treatment in accordance with the invention can cut losses by half or more. Moreover, sheets of similar composition show different losses without inventive treatment, but the method in accordance with the invention tends to equalize the losses, i.e., losses are reduced to approximately the same value in different sheets, and the residual loss is essentially dependent only upon the composition, e.g., the silicon content.

The surface treatment in accordance with the invention changes the mechanism (large Bloch walls and pine tree or dendrite pattern) which tends generally to increase losses in cube texture sheets. In the following it shall be explained, why grooving changes the loss producing mechanism so that the losses are, in effect, cut. Grooves placed as outlined above, partitions a thin surface layer underneath a large grain surface, normally available for these loss increasing mechanisms, and establishes therein many small and separate regions. The depth of the layer so affected is determined by the depth of the grooves. The

thus modified individual pine tree or dendrite patterns in such small regions when taken together occupy considerably less space than the original pattern on the same sheet surface portion without treatment. Moreover, the grooves have the effect that during a magnetization cycle Bloch wall shifting is completed already at a lower induction than in a grain with large, undisturbed surface area. Thus, the losses actually incurred depend primarily only upon the composition of the sheet material and can be regarded at minimum. It should be mentioned that even cube texture sheets, the (100) crystal surfaces of which are not disoriented, still exhibit lower losses when provided with grooves.

It should be mentioned that it is known per se to scratch the surface of unoriented sheets but for an entirely different purpose. Scratching was provided here after rolling for obtaining a uniform, coarse-grained texture after recrystallization. Moreover, the distances between the scratches or grooves in this case was larger than the existing grain size.

The method of the present invention differs from this known treatment of sheet metal in that presently the deformation requires very narrow grooves, provided upon already existing large grains and in a thin surface layer of sheets consisting of oriented crystals. Moreover, the invention is practiced on the finally annealed and recrystallized sheet. Subsequent stress annealing for tension elimination at recrystallization temperature may, but does not have to be, provided. Whether such annealing can take place will be seen after testing small samples.

Practicing the invention does not require particular change or arrangement of the cube texture grains or crystals, even through grains or crystals are usually very large and may cover up to several square centimeters. Also, should the crystals accidentally have elongated shape, there are no particular requirements of the orientation of the longitudinal axis of such grain relative to any other direction, for successfully practicing the invention. Moreover, if first there was hot or cold rolling with subsequent annealing the providing of grooves should not cause recrystallization, particularly primary recrystallization, for obtaining grains or crystals, which are coarser and are particularly oriented as to their long crystallite axis.

The following examples show results of applying the inventive method. Sample strips which recrystallized and exhibited cube orientation subsequent to final annealing, had differently high losses. The losses were measured before and after providing grooves in accordance with the invention. The losses were measured in watts per kilogram material, for 10 kilogauss magnetiaztion (V -value) at 50 Hz. A so-called Forster Ferrograph was used as measuring instrument. The grains in the sample strips had a direction of their [001] edges (or their projection in the plane of the sheet) to be within deviation from the direction of rolling.

EXAMPLE 1 V (initial): 0.89 watt per kilogram material, grooving with cutter, groove distance 0.5 mm. groove depth to 30.10 mm. direction of grooves on one side of sample, transverse direction of rolling V (after grooving): 0.50 w./-kg., corresponding to reduction by 44% of initial value.

EXAMPLE 2 V (initial): 0.85 w./kg.

Three fold grooving of one side with abrasive powder known under designation SiA 100 C wit-h pressure ap- 4 plied, 0.55 kg./cm. grooves had direction transverse to direction of rolling. Single grooving on the other side using similar abrasive powder under similar conditions. V (after grooving): 0.53 W.Cr, corresponding to reduction by 30% of initial value.

EXAMPLE 3 V (initial): 1.07 k./kg. grooving on both sides with cutter groove depth 20 to 30 10* mm. groove distance 0.5 mm. annealing after grooving, 30 minutes at 700 C. V (after grooving and annealing): 0.58 w./kg. corresponding to reduction of losses by 46% of initial value.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constiuting departure from the spirit and scope of the invention are intended to be included.

What is claimed is:

1. A method for reducing ohmic power losses of silicon or molybdenum containing iron alloy steel sheets which comprises providing a completely recrystallized, coarse grained sheet; and partially plastically deforming said completely recrystallized, coarse grained sheet by imparting grooves or serrations to the surface portion on at least .one side of said sheet whereby the domain texture in a surface region of the individual crystallites is locally modilied and surface portions adjacent to the localized deformation remain substantially undeforrned.

2. Method as in claim 1, the deformation step including grooving of the surface of the sheet, the grooves provided having distance from each other considerably smaller than the mean size of oriented sheet elements.

3. Method as in claim 2, the grooves provided at distances from each other in the range from 0.1 to 1.0 mm.

4. Method as in claim 1, the plastic deformation including the providing of grooves having depth up to 4.10 mm.

5. Method as in claim 1, the plastic deformation including the providing of grooves on both sides of the sheet, the grooves on one side thereof oriented parallel to the direction of rolling, the grooves on the other side of the sheet oriented transverse to the direction of rolling.

6. Meth0d as in claim 1, the deformation step being subdivided into sequential deformation steps, there being stress annealing between sequential deformation steps.

7. Method as in claim 1, and including stress annealing subsequent to said deformation step.

8. Method for reducing glossiness of cube texture sheets as in claim 1, the plastic deformation including the providing of grooves at different orientation on both sides, the grooves having distance from each other considerably smaller than the grain size the grooves having depth not deeper than about 0.04 mm.

9. Methods as in claim 8, there being stress annealing at least subsequently to the beginning of the grooving step.

References Cited UNITED STATES PATENTS 2,473,156 6/1949 Littmann 148-111 3,180,767 4/1965 Easton et al. 148-420 3,212,942 10/1965 Takahashi 148-111 X 3,287,184 11/1966 Koh 148-111 X 3,347,718 10/1967 Carpenter et al. 148-111 L. DEWAYNE RUTLEDGE, Primary Examiner G. KJWHITE, Assistant Examiner U.S. Cl. X.R. 148-112, 120, 121 

